The invention relates to thermally removable, polymeric adhesives. More particularly, the invention relates to a method of making thermally removable, polymeric adhesives prepared using the Diels-Alder cycloaddition reaction.
Thermosettable adhesive compositions have been used in a variety of applications where a semi-structural bond between two substrates is required. The semi-structural bond is necessary to ensure that the substrates are inseparable. In most applications, the bond is designed to be permanent. There are applications, however, in which it would be preferable for the adhesive composition to exhibit high performance bond properties during use (i.e., the period and environmental conditions, e.g., temperature range, over which the adhesive composition performs as a semi-structural adhesive), yet be removable after use. A tension exists between these opposing performance criteria. In the aerospace industry, for example, decorative sheets are often adhered to the interior walls of aircraft cabins using thermosettable adhesives. Over time the decorative sheets become marred (e.g., soiled, cut or torn) and styles change. It would be preferable if these decorative sheets could be removed and replaced with new sheets. Following cure, however, substrates bonded together by thermoset adhesive compositions are substantially inseparable. As a result, efforts to separate the substrates are often unsuccessful and result in substrate damage. In addition, the cured adhesive composition exhibits unpredictable cohesive and adhesive failure at either substrate. If the adhesive can be made removable, then the removable adhesives can also be used to form metal-to-metal joints, foam-to-metal joints and foam-to-foam joints that can subsequently be taken apart with no collateral damage to components. These products allow for repairs, upgrades and eventual dismantlement of assemblies.
A variety of thermosettable adhesive compositions are used to form semi-structural bonds to substrates. Thermosettable polyurethane adhesive compositions are often used to bond substrates together. Single package solvent-borne thermosettable polyurethane adhesive compositions rely on atmospheric moisture for curing. Water-borne thermosettable polyurethane adhesive compositions are cured by the addition of water dispersible isocyanate groups to the adhesive composition. The isocyanate groups react with the urea, amino-hydrogen and hydroxyl groups present in the water-borne prepolymer to crosslink the composition. Wesp (U.S. Pat. No. 3,765,972) describes a pressure-sensitive adhesive composition for use wherever a strong permanent bond is desired. The adhesive composition includes a latex and a transient tackifier that includes an epoxy resin and a curing agent. The latex portion of the adhesive composition provides the film-forming capability of the composition.
Recker (U.S. Pat. No. 5,464,902) describes incorporating minor quantities of functionalized, partially crosslinked, elastomeric particles having a glass transition temperature of less than 10xc2x0 C. into epoxy resin systems to toughen the epoxy resin systems against impact-induced damage. The toughened matrix resin systems may be utilized as neat films in structural adhesives or may be scrim supported.
Loder et al. (U.S. Pat. No. 4,049,483) describe a hot melt adhesive system of hot melt adhesive and inherently tacky elastomeric copolymer microspheres. The hot melt system has pressure sensitive adhesive characteristics at room temperature. The patent further describes adding a tackifying agent to the hot melt adhesive system to enhance the room temperature adhesion of the adhesive surface. Once the hot-melt adhesive system has been heat-activated, the adhesive is capable of forming a substantially permanent high strength hot melt bond. The basic properties of the hot melt matrix are unaffected by the inclusion therein of the microspherical adhesive.
Dershem et al. (U.S. Pat. No. 6,034,194) describe an adhesive consisting essentially of a liquid bismaleimide, alkylenes and aromatic bridging groups. The adhesive is a thermoset that is stable to at least 250xc2x0 C. The adhesive is not thermally removable at temperatures below 250xc2x0 C.
Diels-Alder reactions have been shown to produce materials that are thermally removable. For example, Loy et al. (U.S. patent application Ser. No. 09/484,839, now U.S. Pat. No. 6,337,384 B1; incorporated herein by reference) describe epoxide compositions formed by Diels-Alder reactions utilizing bismaleimide and furans to form thermally removable epoxides, but the materials do not possess good removable adhesive properties.